1. Field of the Invention
This invention is generally related to digital imaging systems and more particularly to techniques for detecting a white point and white balancing color images.
2. Related Art
Digital imaging systems are becoming increasingly popular, particularly due to the continuing decline in the cost of digital cameras and personal computers (PCs). Affordability is needed for the continued expansion of the digital imaging market, but the quality of digital color images is also an important consideration for the consumer.
A digital image frame is composed of a number of picture elements (pixels) that together represent the image of a captured scene. Each pixel has at least one pixel value representing the detected intensity of incident light of a particular color that was detected by a corresponding sensor element of an electronic color image sensor. Due to practical limitations in color sensor electronics, however, color images obtained using digital cameras do not accurately display the color content of the scene.
The color of an object recorded by a color sensor typically varies with the light source that illuminates the scene. The true color of the object is captured only if the proper illuminant, having a certain color spectrum, is present. For many sensors, daylight is the proper illuminant, such that an image captured by such a sensor of a white object illuminated with daylight will show the object in its true white color when displayed. However, when the same object is illuminated indoors with a household light bulb, the captured image will show the object as having a slightly reddish color due to the greater red content of light produced by incandescent light bulbs. Similarly, if the white object were illuminated with a fluorescent lamp, then the object in the captured image would have a bluish color. The human eye as an image sensor and the human visual system as a whole can correct for such a color distortion, such that the white object is seen as true white under a wide range of different illuminants. Image sensors, however, cannot inherently make such a correction.
Typically, color correction in imaging devices such as video cameras can be achieved by using certain techniques collectively known in the industry as white balancing. Such techniques have been used or proposed for use with video cameras and digital still cameras. These white balancing methods have been typically implemented as digital logic circuitry aboard the video camera or the digital still camera. Each image frame is white balanced prior to it being transferred outside the camera or stored on recording media such as a video cassette or a removable non-volatile memory.
While the implementation of white balancing in logic circuitry easily allows the transfer of image frames at sufficiently rapid frame rates for display as a video, the dedicated logic circuitry substantially increases the cost of manufacturing the imaging device. Moreover, the conventional white balancing methods referred to above partition an image frame into sections and compute white points and perform white balancing upon each section separately. When the white balanced image is subsequently displayed, undesirable artifacts may appear near the boundaries between such white balanced sections.
It would be desirable to have a white balancing technique that helps reduce the manufacturing cost of the imaging device and helps reduce the incidence of artifacts in an image, while at the same time support a sufficiently rapid frame rate for video.